The present invention relates to a non-metallic connecting device for pipes conducting pressure medium, the non-metallic connecting device being characterized by the capacity to accept an insertable and separable insert to restrain a pipe end within the connecting device, and in particular, the present invention relates to a filament-wound composite connecting device for preparing a restrained joint between large diameter pipes conducting pressure medium.
Potable water is distributed at the lowest cost per mass of any commodity. Traditional water distribution systems employed large diameter metallic pipes interconnected by joints featuring complex metallic couplings. Traditional metallic pipes are subject to corrosion from some potable waters. Metallic couplings are also subject to failures, not just from corrosion but also from mechanical failure in response to both radially and axially directed forces. Such joint disrupting forces originate in frequent and occasionally substantial fluctuations in pressure and/or water flow. Traditional metallic coupled joints are often supported against separation or movement by association with strategically arranged masses of concrete. The concrete masses restrain the metallic coupled joints to counter and resist undesirable shifts that might cause a joint to mechanically fail. Preparation of each metallically coupled joint along with the often required concrete mass restraining the mechanically coupled joint is particularly problematic. Traditional water distribution systems are increasingly viewed as needing a fresh look with respect to efficiency, durability, capital cost and ease of installation and maintenance.
Modern materials are now being substituted for the metallic pipes and metallic mechanical couplings in many water distribution systems. Plastic pipes are not subject to corrosion. Joining plastic pipes, however, remains a challenge. In particular, plastics often have a slick or slippery surface relative to surfaces on metallic materials. Polyvinyl chloride (PVC) and polyethylene (PE) are two plastic materials particularly considered feasible as pipe materials for large diameter pipes. Each of these materials, however, comes with its own challenges in being acceptable as a large diameter pipe material. As will be explained, these challenges often involve difficulties in joining pipes to form a distribution system.
Large diameter pipes systems of PE, and in particular, high density polyethylene (HDPE), are able to function at 200 psi and, therefore, tend to dominate the plastic high pressure water system market. However, HDPE large diameter pipes are relatively expensive. Moreover, connecting two HDPE large diameter pipes is both costly and time consuming. Each HDPE joint requires performing a fusion welding process. The fusion welding process requires about an hour to complete each joint and further requires the on-site presence of both significant large equipment and skilled labor. Providing sufficient working space about the joint while performing fusion welding is yet another concern. Attempts to substitute unskilled labor can result in defective fusion HDPE joints characterized by unacceptable axial tensile strength.
PVC pipe is relatively less expensive than HDPE pipe. As such, it might initially seem an excellent choice for a distribution system. The challenges of PVC pipe distribution systems can be readily understood by considering a 16 inch diameter PVC pipe system, exemplary of a large diameter pipe system. Currently available PVC 16 inch diameter pipes are capable of containing and operating at pressures of 200 psi. Currently available PVC 16 inch diameter water distribution systems, however, are operated at only 90 psi due to the limited pressure rating of the current PVC pipe couplings used in preparing joints between PVC pipes. The inventor has investigated extensively and discovered poor pressure performance of the current PVC pipe couplings relates to failure to withstand the hoop stresses associated with higher pressures, such as in the ASTM D2241 qualification test. Susceptibility to high pressure might be caused by insufficient wall thickness of the couplings about the internal O-ring grooves. That is, each currently available PVC coupling includes a pair of internal O-rings and a groove holding each O-ring. When part of a joint, the internal portion of the PVC coupling between the O-rings is subject to system pressure. System pressure acts upon this internal portion to radially expand a hoop shaped portion of the PVC coupling adjacent to each of the O-ring grooves. As higher system pressures further expand the PVC coupling adjacent to the O-rings, a leak occurs at one of the two O-rings. Such an undesired expansion thereby results in a water distribution system failure and, therefore, is the cause of the lower 90 psi pressure limitation.
Current 16 inch PVC couplings are typically manufactured via a two-step process. The first step of the process entails extruding a thick walled PVC billet (i.e. a thick walled hollow cylinder). A pipe stop, O-ring grooves, and spline grooves are then machined internally from the cylindrical billet to produce a finished PVC coupling. Because of the design of the coupling, the central pipe stop for a 16″ pipe must remain after machining away PVC material from the total starting thickness of the billet. The remaining wall thickness of the finished coupling after machining is only 0.635 inches. The O-ring groove depth cut into this 0.635 inch wall is 0.295 inches. This leaves a wall thickness of only about 0.34 inches between the O-ring groove location and exterior surface of the cylindrical coupling. As noted above, this thickness is insufficient to resist system failure at 200 psi.
The inventor investigated further by experimenting with a number of apparently simple solutions in an attempt to discover a more pressure resistant PVC coupling for PVC pipe. The desired new PVC coupling would allow the 200 psi high pressure capacity of PVC 16 inch pipe to be exploited rather than settling for a distribution system derated to 90 psi. For example, a simple solution might seem to be manufacturing a PVC coupling having sufficient wall thickness to withstand the hoop stresses generated in the ASTM D2241 testing. However, the starting wall thickness of the necessary billet would need to be 2 inches or greater for such a simple solution. One constraint that should also be noted is that only certain PVC extrusion mixtures have regulatory approval for potable water service. Initial experiments based upon extruding a billet with a 2 inch wall thickness, for an experimental 12 inch diameter PVC coupling of an approved extrusion mixture, necessitates that residence time of the PVC material in an extruder increase to a duration time that results in scorching or burning of the PVC resin in the resulting experimental billet. Such scorching or burning unacceptably degrades the physical properties of the PVC material. Moreover, it is thought that excessively thick extrusions fail to achieve a desirable level of “fusion” and thus are plagued by insufficient strength. It was concluded that an experimental coupling prepared from an approved PVC mixture as a billet with 2 inch thick walls of degraded PVC material would not pass the performance tests indicated in ASTM D2241 and would not yeild a water distribution system having the desired performance reliability.
Another attempted simple solution might seem to be providing additional reinforcement to current PVC couplings. Prior attempts to constrain the above-mentioned radial expansion in current PVC couplings with supplemental bands or rings of appropriately situated fiberglass reinforcement have failed due to underlying expansion and eventual bursting of the underlying PVC adjacent an O-ring groove when tested at appropriate internal pressure. The inventor concluded that the desired PVC based coupling was not the correct approach and began to explore other coupling materials.
Yet another simple solution might seem to be available by reinforcing current HDPE pipe couplings by supplemental filament winding layers over a HDPE pipe coupling. However, experimental reinforced HDPE couplings demonstrated unacceptable axial tensile strength, presumably due to the inherent poor compressive strength of HDPE. This approach was discarded after observing that a vertical face of a spline groove in the HDPE pipe coupling had sheared out in response to axial stress.
Still another simple solution involved compression molding two halves of a glass-filled polypropylene liner, butt welding the two halves of the liner together and then filament winding layers of Twintex® glass fiber roving over the newly-formed liner for added hoop strength. The butt welding of the two halves proved to be not feasible as glass fibers migrated to the surface edge during the heating operation. The presence of the glass made it impossible to form a leak proof bond in the butt welded coupling.
Another approach involved compression molding of Owens Corning Azdel® composite material as a liner and bladder molding Twintex® fabric around the liner for greater hoop strength. This approach failed because good consolidation was not achieved and water migration through the coupling wall was observed during hydrostatic testing.
Yet another approach involved winding 60% Twintex® glass roving into a hollow cylinders and subsequently machining internal grooves for splines and O-ring. This approach failed because of water permeation through the walls of the experimental couplings. The water permeation was interpreted as the interior surface of the experimental coupling becoming resin starved during consolidation of the Twintex® rovings. Furthermore, in addition to the unacceptable water permeation characteristic, machining of the spline and O-ring grooves proved a challenge and, when finally machined and tested, the experimental couplings still tended to split circumferentially.
Tubular items of composite prepared by filament winding processes have been used in very limited situations as pipe systems. In U.S. Pat. No. 3,606,402, Medney discloses precision machined male and female integral ends on filament wound pipe lengths being mated and locked by insertion of paired keys into a locking groove. In U.S. Pat. No. 3,606,403, Medney discloses an adhesive joint of male and female integral ends on filament wound pipe lengths. Such joining arrangements require that the pipes themselves are filament wound and do not take advantage of the more economical PVC pipes.
Clearly, an improved coupling for PVC pipe was still needed and a simple solution did not appear available. The inventor then discovered a more elegant and sophisticated solution, explained subsequently herein. The present invention addresses and overcomes the deficiencies of the current PVC coupling, allowing greater internal pressure without failure. This in turn allows for more efficient use of the high pressure capabilities of large diameter PVC pipe. Additionally, the present invention is less expensive and time consuming than current HDPE joint preparation by fusion welding. Moreover, a joint assembled with the present invention allows some unexpected new applications of PVC pipe. Further, the present invention enables large diameter PVC pipe water distribution systems having improved systems reliability.